1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) giant magnetoresistive (GMR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP-GMR sensor with magnetic damping to suppress spin transfer torque (STT).
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu) or silver (Ag). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
CPP-GMR sensors are susceptible to current-induced noise and instability. The spin-polarized bias or sense current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque (STT) on the local magnetization. This can produce magnetic instabilities and even continuous gyrations of the magnetization in the ferromagnetic layers, resulting in substantial low-frequency magnetic noise in the measured electrical resistance if the bias current is above a certain level. This effect is described by J.-G. Zhu et al., “Spin transfer induced noise in CPP read heads,” IEEE Transactions on Magnetics, Vol. 40, January 2004, pp. 182-188. To maximize the signal and signal-to-noise ratio (SNR) in CPP-GMR sensors, it is desirable to operate the sensors at a high bias current density. However, the adverse effect of STT limits the bias current at which the sensors can operate. Both the free layer and reference layers in the sensor are susceptible to STT, and therefore the layer with the highest sensitivity to STT will typically limit the performance of the sensor. One proposal to alleviate this problem to some degree is to increase the magnetic damping of the ferromagnetic layers, i.e., to increase the effective thermal coupling between the magnetization (spin-system) and that of its host lattice. With sufficient damping, the magnetic layer with magnetization excitations caused by STT will lose energy to the lattice faster than it can absorb energy from the bias current via STT.
U.S. Pat. No. 7,423,850 B2, assigned to the same assignee as this application, describes a CPP-GMR sensor with an antiparallel free layer (AP-free) structure, i.e., two free layers with magnetizations oriented antiparallel across a Ru spacer layer, wherein one of the free layers includes a NiFeTb film for magnetic damping of the other free layer across the Ru spacer layer. U.S. Pat. No. 8,233,247 B2, assigned to the same assignee as this application, describes a scissoring-type CPP-GMR sensor wherein each of the two free layers is in contact with a magnetic damping layer formed or Pt, Pd or a lanthanoid.
However, among the most vulnerable parts of the sensor to STT are the magnetic layer edges where canted or loose spins may be more readily excited due to their non-collinear orientation with either the free layer or the pinned layer. What is needed is a CPP-GMR sensor with increased magnetic damping at the sensor edges to suppress STT at the most sensitive areas of the sensor without reducing the sensor signal near the center of the sensor.